In the past, downhole well formations have been treated with a highly viscous thixotropic treating fluid in order to introduce the treating fluid into the formation, as set forth, for example, in U.S. Pat. No. 4,215,001 to Elphingstone et al. The highly viscous fluid having thixotropic properties is prepared from an aqueous alkali metal silicate solution. The aqueous alkali metal silicate solution is prepared by mixing about five parts by volume grade 40 sodium silicate solution with about 95 parts by volume water. To this solution is added an aqueous acid solution while agitating the mixture. This lowers the pH of the mixture to a value in the range of 7.5-8.5, whereby the alkali metal silicate polymerizes to form a highly viscous rigid gel. This highly crosslinked rigid gel structure is not soluble in water, but is gelatinous due to water being entrapped in the polymer structure. In order to impart thixotropic properties to the polymerized silicate gel, it is sheared by mixing or agitation, preferably while the polymerization reaction is taking place. The resultant substance exhibits thixotropic properties, that is, a low viscosity in turbulent flow but a high viscosity when at rest or at low shear rates.
The viscosity of the polymerized silicate gel can be increased by the addition of a gelling agent, such as hydroxyethyl cellulose (HEC), carboxymethyl cellulose (CMC), hydroxypropyl guar (HPG), and carboxymethyl hydroxypropyl guar (CMHPG), thereto which hydrates with free water contained in the gel.
The Elphingstone et al U.S. Pat. No. 4,231,882 teaches a highly viscous thixotropic treating fluid for treatment of a subterranean well formation, and is commonly referred to as "fracturing fluids". The treating or fracturing fluid is formed by combining an aqueous acid solution with an aqueous alkali metal silicate solution and a gelling agent whereby a highly viscous polymerized alkali metal silicate gel of low pH is produced, followed by shearing of the gel to impart thixotropic properties thereto.
Reference is made to the Elphingstone et al patents for further background.
Applicant has discovered that a polymerized alkali metal silicate gel is useful as a drilling fluid, and especially as a drilling fluid when drilling through a high temperature strata of the earth, as for example, in geothermal wellbores. A gel system that uses a drilling fluid as described herein is inexpensive and non-damaging to the production formation; it has excellent high temperature stability; and should the gel be lost to a fractured formation, it is easily removed by contact with mixtures of hydrochloric and hydrofluoric acids.